1. Field of the Invention
The invention relates to microwave heating and apparatus for producing broiled surfaces on a radiated load.
2. Description of the Prior Art
The heating of articles by microwave energy has become widely accepted both here and abroad in recent years due to the rapid preparation times resulting from the so-called "dielectric heating phenomenon." Such apparatus typically includes an energy source such as a magnetron with the energy fed within a conductive wall oven enclosure through waveguide transmission means. The electromagnetic waves are radiated and reflected within the enclosure in free space and are distributed by such means as mode stirrers to uniformly surround and be absorbed by the load to result in high frequency oscillatory movements of the molecules to cause heating by molecular friction. The allocated frequencies for such heating apparatus are assigned by the Federal Communication Commission and are 915.+-. 13 MHz and 2450.+-. 50 MHz. The term "microwaves" is intended to define electromagnetic energy radiation having wavelengths in the order of 1 meter to 1 millimeter and frequencies in the order of 300 MHz to 300 GHz.
All materials exposed to electromagnetic radiation have differing dielectric constant and loss tangent characteristics and, therefore, the rate of heating becomes a varying factor. Accordingly, the varying absorption of the radiated energy causes the depth of penetration and surface coloration of loads to vary. Where a browned surface is desired, similar to broiling, this is difficult to achieve without long exposure which results in overcooking of the interior regions because the microwave oven enclosure is cooler relative to electric and gas apparatus so that the exterior load surfaces tend to be cooler than the interior and there is heat loss due to evaporation of moisture. Microwave cooked exterior surfaces, therefore, have been treated in a slightly different manner to produce the desirable coloration.
Prior art techniques for browning include the incorporation of electric or gas broiling elements in the microwave oven. Another method involves the coating of the outer surfaces with a food additive having a higher energy absorbing characteristic which will lead to more rapid heating of the outer surfaces, while the remainder of the load achieves the desired degree of cooking. Still another example of prior art teachings involves the use of lossy ovenware or utensils having a selective heating capability by means of the use of conductive materials. Such conductive materials may be incorporated in a shelf of a dielectric material supporting the load or comprise radiating means having a plurality of rods with a pre-determined spacing to cause the rapid absorption of the microwave energy by the rods as shown in U.S. Pat. No. 3,591,751, issued July 6, 1971 to C. E. Goltsos U.S. Pat. No. 2,830,162, issued Apr. 8, 1958 to D. A. Copson et al., and assigned to the assignee of the present invention, utilizes a utensil of a ferromagnetic material responsive to the energy impinging thereon up to the Curie temperature point. Thereafter, the utensil becomes substantially nonresponsive and is pervious to the electromagnetic energy. Ferromagnetic materials sustain the alternating electric and magnetic fields and, characteristically have high energy loss at the temperatures below the Curie point. Such materials include alloys of manganese, tin and copper, or manganese, alumnium and copper as well as alloys of iron and sulphur, such as pyrrhotite, whose crystals have the form of hexagonal prisms. Examples of such materials are zircomates of lead and barium and the titanates of lead, barium and strontium.
Other examples of the prior art are found in U.S. Pat. No. 3,219,460 issued Nov. 23, 1965 to E. Brown which discloses containers of a dielectric material having electrically conductive shields such as aluminum wrapped completely around the side and bottom surfaces. Other geometric patterns are achieved in the aluminum foil. U.S. Pat. No. 3,302,632 issued Feb. 7, 1967 to E. C. Fichtner also relates to a plastic-type package having varying microwave transparent characteristics. Such containers are readily adaptable to the preparation of frozen foods, such as T.V. dinners.
The prior art radiant heaters, selected heating packaging and lossy utensils have been utilized, however, a need still exists for a simple, less costly apparatus which will not be subject to breakage; require additional cleaning or reduce the amount of power available for heating.